We have cloned and functionally characterized an Na+-coupled citrate transporter from Caenorhabditis elegans (ceNAC-2). This transporter shows significant sequence homology to Drosophila Indy and the mammalian Na+-coupled citrate transporter NaCT (now known as NaC2). When heterologously expressed in a mammalian cell line or in Xenopus oocytes, the cloned ceNAC-2 mediates the Na+-coupled transport of various intermediates of the citric acid cycle. However, it transports the tricarboxylate citrate more efficiently than dicarboxylates such as succinate, a feature different from that of ceNAC-1 (formerly known as ceNaDC1) and ceNAC-3 (formerly known as ceNaDC2). The transport process is electrogenic, as evidenced from the substrate-induced inward currents in oocytes expressing the transporter under voltage-clamp conditions. Expression studies using a reporter-gene fusion method in transgenic C. elegans show that the gene is expressed in the intestinal tract, the organ responsible for not only the digestion and absorption of nutrients but also for the storage of energy in this organism. Functional knockdown of the transporter by RNAi (RNA interference) not only leads to a significant increase in life span, but also causes a significant decrease in body size and fat content. The substrates of ceNAC-2 play a critical role in metabolic energy production and in the biosynthesis of cholesterol and fatty acids. The present studies suggest that the knockdown of these metabolic functions by RNAi is linked to an extension of life span and a decrease in fat content and body size.
Abbreviations used: NaDC, Na+-coupled dicarboxylate carrier/transporter; ceNAC-2, Caenorhabditis elegans Na+-coupled citrate transporter; drIndy, Drosophila Indy; NaCT, Na+-coupled citrate transporter; GFP, green fluorescent protein; HRPE, human retinal pigment epithelial; dsRNA, double-stranded RNA; cRNA, complementary RNA; RNAi, RNA interference; TCA, tricarboxylic acid; TEVC, two-electrode voltage-clamp.
The ceNAC-2 cDNA and its encoded transporter protein sequences have been deposited in the GenBank Nucleotide Sequence Database under accession number AY090486.